Electric furnace



J n 1. 4 R. A. WILKINS ET AL 2,680,144

ELECTRIC FURNACE Filed April 14 1951 3 Sheets-Sheet l w Jo4 9W W vMALI/aw w June 1. 1954 w l s ETAL 2,680,144

ELECTRIC FURNACE Filed April 14, 1951 3 Sheets-Sheet 2 mZon-fi m? a Earzfi E1 0 land v M'LALM ,5-

June 1. 1954 R. A. WILKlNS ET AL 2,680,144

ELECTRIC FURNACE Filed April 14, 1951 s Sheets-Sheet s Q mm 17 wjiawvamhr Patented June 1, 1954 UNITED STATES PATENT OFFICE ELECTRICFURNACE Maryland Application April 14, 1951, Serial No. 221,028

Claims.

Our invention relates to electric furnaces which, amon other uses, maybe used for practising the methods Of smelting zinc described inco-pending application of Richard A. Wilkins and Kenneth A. PhillipsSerial Number 220,5'7 i, filed April 12, 1951. The present applicationfrom the aspect of a zinc smelting electric resistor furnace having agraphite partition for protecting graphite heating resistors of thefurnace is a continuation-in-part of applicants co-pending applicationSerial Number 93.954, filed May 18, 1949.

In the method described by the Wilkins and Phillips application SerialNumber 220,574 above referred to roasted zinc ore concentrate containingzinc oxide and impurities is mixed in granular form with reaction carbonin granular form and the mixture charged to a hearth in an operativelyclosed chamber to form on the hearth a mass of the charge having anupper downwardly sloping free surface which at its lowermost portion issubstantially in the plane of the hearth. This reaction carbonpreferably is in the form of coke or anthracite coal rendered granularby a crushing or grinding operation, or coke breeze, or bituminous coalfines. The mixture is fed to the chamber from below the sloping surfaceof the mass of charge and adjacent its higher portion so that the chargebeing fed forces the material of the mass to this sloping surfacesubstantially throughout its entire extent and thus also acts to moveaway from the body of said mass the material of the mass adjacent thelower edge of said sloping surface. On the mass of charge on the hearthheat is radiated downward on said sloping surface to heat the surfaceportion of the mass to a temperature which will react the zinc oxide atsuch portion with the carbon. The zinc produced is in the form of avapor which, together with the carbon monoxide and other gases formed bythe reaction and heat, escapes upward from the free surface portion ofthe mixture. These escaping vapors and gases agitate the upper surfaceportion of the mixture and cause the excess coke, coal and other solidparticles, with molten slag and iron droplets adhering to them, togravitate down the sloping surface to its lower edge portion for removalfrom the chamber. The zinc vapors and gases which escape upward from themass may be discharged from the chamber into a condenser where they arecooled for condensing the vaporous zinc to liquid zinc.

The furnace according to the present invention comprises electricheating resistors, which are 2 preferably of graphite, for radiatingheat downward on the charge. These resistors are heated toincandescence, and, when of graphite or the like, are susceptible tochemical attack by dust and gases emanating from the charge if such dustand gases are of such composition, as, for example, Zinc oxide dust andcarbon dioxide, that they will react with carbon. The present inventionhas among its objects protecting the resistors from such attack.

In the drawings:

Fig. 1 is a central longitudinal vertical section, on the line I-I ofFig. 2, with parts in elevation, of a furnace according to theinvention;

Fig. 2 is a section on the line 22 of Fig. 1; and

Fig. 3 is a section on the line 3-3 of Fig. 2, with parts omitted.

Referring to the drawings, the furnace comprises an open top casing l ofsheet metal, preferably steel, having side walls 3, 5, I and 9 and abottom wall ll all welded together to form a fluid tight construction.As shown, this casing is provided with a cover 53 comprising areinforced metal plate [5 the bottom surface of which rests on the upperedges of the side walls of the casing. The cover is removably secured tothe casing by the bolts ll welded at their lower end portions tobrackets [9 (Fig. 2) secured to the stiffening bars 2| for the sidewalls 7 and 9 of the casing, to which latter the inner edges of thesebars are welded.

Extending continuously around the upper edge portions of the side wallsof the casing are angle-irons 23 the edges of the horizontal webs ofwhich are welded to the casing side walls in a fluid tight manner toform a trough 25 which is filled with sand, oil or the like and intowhich projects a flange 2'! extending continuously around the edge ofthe cover plate 55 at its under side in fluid tight welded relationthereto. In this way the cover is secured to the casing in a fluid tightmanner.

Internally the casing 4 contains a structure which forms a furnacechamber having, an upper portion 29 and a lower portion 3|. The bottomof the lower portion 3| is built up of hard carbon blocks 33 to form ahearth 35, which hearth, as shown by Fig. 2, is of arcuatecross-section. The blocks 33, as shown by Fig. 1, extend from one end ofthe furnace chamber to a point spaced from its opposite end to form awell or pit 3? having a bottom wall 39 formed by the hard carbon blocks4|.

At opposite longitudinal sides of the hearth the blocks 33, as shown byFig. 2, abut the wedge- 3 shaped blocks a3 a row of which extends forthe entire length of the chamber. On the upper fiat surfaces of thesewedge-shaped blocks are supported at opposite sides of the chamber theelongated horizontal graphite slabs ,55, similar slabs l! of graphitebeing positioned at opposite ends of the chamber. These slabs 35 and ilare grooved at their upper sides as shown at 49, these groovescommunicating with each other at the four corners of the chamber. Thegrooves in the slabs 45 receive the lower edge portions of verticallypositioned horizontally extending graphite slabs El, and the grooves inthe slabs ll receive the lower edge portions of vertically positionedhorizontally extending graphite slabs 53, the slabs 5i and 53 abuttingat the four corners of the chamber. Supported by each of the slabs 5!and the end portions of the slabs 3 at their upper edges are graphitebars 55 which,

as shown by Fig. 3, extend for the length of the chamber, whilesupported by each of the slabs 53 at their upper edges are graphite bars5! which extend transversely of the chamber from one bar 55 to the otherin abutting relation therewith, the bars 55 and 5'! being grooved attheir lower sides, as shown at 59, for receiving the upper edges of theslabs 5i and 53;. Resting on the upper sides of the bars 55 arevertically positioned horizontally extending graphite slabs 6|, Whileresting on the bars 5'! and the end portions of the bars 55 arevertically positioned horizontally extending graphite slabs 653, theseslabs 5i and 53 abutting each other at the four corners of the chamber,and the upper sides of the bars being grooved, as shown at 55, forreceiving the lower edges of the slabs. The roof of the chamber isformed by the laterally abutting elongated graphite slabs l5? which restat their opposite end portions on the top edges of the slabs M, the edgeportions of the slabs El adjacent the slabs 63 also resting on the topedges of the latter.

The carbon blocks 33 and ll forming the hearth 35 and the bottom wall 39of the well or pit 31, respectively, are supported on a layer formed bycourses of firebricks 69, which layer at the sides of the wedge-shapedblocks 3 and adjacent the longitudinal ends of the casing l is builtupward to support and back the slabs 35 and il and to form firebricklayers H adjacent the side walls of the casing. 011 the tops of thefirebrick layers l'l rest the elongated graphite slabs 73 which closethe space surrounded by said layers. The space between these slabs andthe furnace chamber roof slabs ill, and the space between the firebricklayers H and the slabs and bars forming the interior side walls of saidchamber, are filled with a mass of heat insulating material 75 as, forexample, carbon beads or a mixture of the same and lampblack or brokenup charcoal.

Between the slabs 73 and the cover 53 of the casing l, and between thatcasing and the fire-- brick layers adjacent its sides and bottom, areinserted heat insulating packing layers 16 of material capable ofresiliently yielding under high pressure to permit the furnace casingand the interior walls of the furnace to expand and contractindependently of each other so as to prevent rupture of the casing.

In the upper portion 29 of the furnace chamber is positioned a graphiteresistor grid comprising a row of parallel elongated heating resistorbars '11 of graphite. The opposite end bars of the row have extensions79 projecting through the furnace walls in insulated relation thereto,and at the exterior of the furnace these two extensions are providedwith terminals BI for the cables for energizing the resistors, theresistors being connected at opposite ends for series flow of currentthrough them. The resistors are suspended from the roof slabs tl inelectrically insulated relation thereto and the rest of the furnace bythe elongated graphite bars 83. The construction of the resistors andtheir insulated suspension is more fully described in Poland UnitedStates Patent 2,472,613, issued June '7, 1949, and in pending UnitedStates application of Richard A. Wilkins Serial Number 162,220, filedMay 16, 1950, and need not be further described here. The resistor whenheated become incandescent and radiate heat downward toward the hearth.They also act to heat the graphite roof or" the chamber to incandescenceto cause it also to radiate heat downward toward the hearth.

As shown, interposed between the resistor grid and the hearth, inout-of-contacting relation with said grid and its suspension, is a rowof laterally abutting elongated bars 85, of circular crosssection,formed of graphite which is extremely heat refractory and an excellentconductor of heat. These bars form a partition dividing the furnacechamber into its upper and lower portions 29 and 3!. They serve toprotect the resistors from attack by zinc oxide dust, vapors and gasesemanating from the heated charge. For electrical and economic reasons itis highly desirabl to protect the graphite resistors and graphiteresistor suspensions from deterioration, and the relatively inexpensivebars not only act mechanically to protect them from deterioration but doso chemically for the reason that the bars, like the resistors andresistor suspensions, are heated to incandescence and any carbon dioxideemanating from the charge and tending to leak by the partition formed bythe bars must contact with the latter and by reaction with them beconverted to carbon monoxide, which is inert with respect to graphite,before it can reach the resistors and resistor suspensions. The circularcrosssection of the bars gives the upper and lower sides of thepartition a, corrugated shape which increases the area of those sidesand therefore increases the area presented by the lower side of thepartition for contact with carbon dioxide and increases the facilitywith which the partition absorbs the heat from above and radiates suchheat downward toward the hearth.

As shown, the bars 55 are formed to provide shelves ill on which theends of the bars 85 rest, these shelves being inclined downward from onebar 52 to the other so that when the bars 85 are placed on the higherends of the shelves they will roll downward to the lower ends of theshelves and there rest against the abutments 89 formed by the bars 5'!at said lower ends. The bars 85 may be inserted and removed from thefurnace without cooling it. For this purpose the walls of the furnaceare provided with openings in one of which adjacent the higher end ofthe inclined shelves 8? is positioned a graphite sleeve Ell (Fig. 4) andin another of which adjacent the lower end of those shelves ispositioned a graphite sleeve $3. These sleeves are received at theirinner ends in the enlarged diameter portions of openings ill in theadjacent bar 55. The sleeves are normally plugged with removablestoppers in the form of bars 99 preferably of hard carbon which is arelatively poor heat conductor, these bars being preferably of suchlength as to extend for the full length of the sleeves so as to minimizeheat losses from the chamber. As shown, those ends of the sleeves 9I and93 which project from the outer side of the furnace are surrounded bythe water jacketed metal sleeves I60 welded at one end to the casing I,which sleeves I keep the outer ends of the carbon bars 99 cool enough tobe conveniently handled. Conveniently the plugs are provided at theirouter ends with tapped holes I02 for detachably securing to them a rodhaving a threaded end portion adapted to be screwed into said holes sothat the plugs by use of such rod may be pulled from the sleeves 9i and93 when desired. Upon removal of the plug from the sleeve 93 the bar 85at the lowermost ends of the inclined shelves 81 may be pushed out ofthe furnace through such sleeves by means of a metal rod inserted andpushed through the graphite sleeve IIlI of small diameter borepositioned in an opening in the furnace wall opposite the sleeve 93 inalignment therewith, this sleeve IBl communicating with an opening I93formed in the adjacent bar 55. The bore of the sleeve iIlI may benormally plugged with a stopper in the form of a carbon bar similar toone of the bars 99. Ordinarily however, due to the small diameter of thebore of the sleeve IBI, it will suflice if such bore is closed by ametal cap I05 screwthreaded on or otherwise removably carried by a metalsleeve I0! which surrounds the outwardly projecting end of the sleeveIBI and is welded to the adjacent wall of the casing I. For supportingthe bars 85, when they are being inserted and removed from the furnacechamber, the bars 5? are each shown as formed with a horizontal shelf M9on which the bars slide for preventing them from tilting and fallingdownward into the lower portion of the furnace chamber.

After a bar 85 is inserted through the sleeve 9i at the higher ends ofthe inclined shelves 8? it may be readily rolled off the adjacent shelfI09 onto such inclined shelves by use of a suitable metal bar (notshown) inserted through the sleeve 9|, such sleeve having a pointed endwhich may be caused to enter the space between the bar 85 and adjacentvertical wall of the adjacent bar 5'! for prying the bar off said shelfI99. It will be understood that upon removal of a bar 95 at the lowerends of the inclined shelves 8? the remaining bars will roll down saidshelves to place the next consecutive bar opposite the sleeve 93 andthus leave a space at the higher ends of such shelves into which anotherbar 85 may be inserted through the sleeve 9! when the stopper bar 99 inthat sleeve is removed. In this way the bars may be removed forinspection and a damaged bar replaced by a new bar.

As further shown, the furnace is formed with a tap hole II Icommunicating with the well or pit 31, this tap hole being normallyclosed by a removable plug II3 of fireclay or the like, upon removal ofwhich the material which enters the well from the hearth may be tappedtherefrom as a liquid mass or be raked therefrom depending upon thetemperature of such mass and the amount of solid particles containedtherein.

Also, preferably, the exterior of the casing I at its lower portion issurrounded by a water jacket I I5 which has the effect of cooling theexposed surface of the hearth and well 31 so that there will be formedthereon a rather thin layer of solid iron or slag which protects thecarbon blocks forming those surfaces from destruction. The coolingeffect of the water jacket is so coordinated with the thickness and heatconductivity of the materials forming the adjacent portion of thefurnace walls that when molten slag or iron contacts the surfaces of thehearth and well it will solidify and its thickness increase until itssurface temperature reaches the melting point of the slag or iron,whereupon the protective layer thus formed will not build up farther,or, if the slag or iron when it first contacts these surfaces of thehearth and well eats them away, the consequent slight thinning of theblocks presenting those surfaces will cause such surfaces to reach atemperature which will solidify the slag or iron contacting them, whichsolidified slag or iron will build up to form a protective layer untilthe thickness of such layer is such that its exposed surfaces are attheir melting points.

As shown, the vertical wall of the furnace adjacent the end of thehearth opposite the well 31' comprises a sleeve II! of hard carbonhaving a tapered bore II9. At its outer end this bore is provided with ametal sleeve extension I2I communicating with the horizontal conduit I23at the exterior of the furnace. This conduit preferably communicates atone end with the bottom discharge opening of a hopper (not shown).Extending lengthwise of the interior of the conduit is an elongatedrotary screw conveyor I31 driven by an electric motor (not shown). Themixture of oxidized concentrate and coke or coal may be entered into thehopper so as to discharge into the conduit I23 through which the screwconveyor forces the material and causes it to pass through the bore I I9 to feed such material to the furnace and form on the hearth a pile ormass I3! (Fig. 1) having a downwardly sloping free upper surface I39 theslope of which is determined by the angle of repose of the material.This material builds up at its higher end to above the bore H9, and theincoming material from said bore acts to force the material toward thesloping surface of the pile substantially throughout the entire extentof said surface and to move the residue at the lower end of the pilealong the hearth in a direction which is away from said bore.

The vaporous zinc admixed with the gaseous products of the reactionescape from the furnace chamber through an opening I49 in the furnacewall, from which opening they are led through a conduit I5I to acondenser (not shown) where the vaporous zinc is condensed to liquidzinc.

For insuring that no reaction of the zinc oxide and carbon will occur inthe bore I I9 the portion of the water jacket I I5 at the end of thefurnace adjacent that bore is extended upward to form a portion I58 thatsurrounds the sleeve extension IZI of the bore, and by absorbing heatfrom the adjacent portion of the furnace walls acts to keep the materialin said bore and extension at a temperature below that at which anysubstantial re action between the zinc oxide and carbon occurs. Forpreventing zinc vapors from being forced from the furnace chamberthrough the charge into the bore H9 and charging conduit I23, and intosaid bore and conduit when the charge is being initially entered intothe preheated furnace or when the mass of charge on the hearth isotherwise at a low level, in which bore and conduit such vapors wouldcondense to troublesome solid metallic zinc and interfere with thecharging operation, the block or sleeve II? is shown as formed with apassage I59 (Fig. 1) which opens into the bore II9 of said block.Through this passage an inert gas, such as nitrogen, may be fed to thebore II9 from a supply pipe ISI communicating with said passage, theflow of nitrogen being controlled by a valve itS in saidpipe. The amountof nitrogen so admitted to the bore He may be regulated by the valve tocause the pressure in that bore to be slightly higher than that in thefurnace chamber so as to prevent backflow of the zinc vapors into saidbore and the conduit i215. Nitrogen from this passage may also beadmitted to the furnace chamber and hence to the condenser fordisplacing the air contained in them prior to heating the resistors andfurnace chamber preliminary to initiating the smelting operation.

It will be understood that within the scope of the appended claims widedeviations may be made from the forms of furnaces described, withoutdeparting from the spirit of the invention.

We claim:

1. Apparatus for recovering zinc from zinciferone material comprisingwalls forming a chamber, a partition extending across said chamber fordividing it into upper and lower parts, which lower part is adapted tocontain a mixture of zinc vapors and gases containing substance reactivewith graphite, the partition being formed of elongated laterallycontacting round bars and of such extent as to prevent communicationbetween said parts of said chamber except such as may unavoidably occurthrough such interstices as may exist between said bars, a generallyhorizontal heating resistor grid of graphite in said upper part of saidchamber for radiating heat on said partition for heating it toincandescence, said grid comprising a row of interconnected elongatedresistors, the end resistors of said row having extensions projectingthrough said walls in electrically insulated relation thereto, meanssuspending said grid in said upper part of said chamber in spacedrelation to said partition and in electrically insulated relation tosaid walls, the bars forming said partition being of graphite forrendering the partition highly heat conductive and for reacting withsaid substance of said mixture for rendering such of said mixture as mayunavoidably pass through the interstices between said bars chemicallyinert with respect to attack on said grid.

2. Apparatus according to claim 1 in which parallel shelves are formedon a pair of opposite side walls, respectively, of the chamber, on whichshelves the opposite end portions of the round bars forming thepartition rollingly rest, the walls of the chamber having provision forsuccessively entering said bars into the chamber and placing them onsaid shelves at one portion of the latter and for removing them fromsaid shelves to the exterior of the chamber at another portion of saidshelves.

3. Apparatus according to claim 1 in which parallel shelves are formedon a pair of opposite side walls, respectively, of the chamber, on whichshelves the opposite end portions of the round bars forming thepartition rollingly rest, the walls of the chamber having provision forsuccessively entering said bars into the chamber and placing them onsaid shelves at one portion of the latter for removing them from saidshelves to the exterior of the chamber at another portion of saidshelves, the shelves being downwardly inclined from the first mentionedportion thereof to the last mentioned portion thereof for causing saidbars when placed on said shelves at said first mentioned portion to rolldown said shelves to said last mentioned portion.

4. A zinc smelting furnace formed to provide a reaction chamber, apartition formed of laterally abutting round bars of graphite extendingacross said chamber acting to divide it into upper and lower parts, saidpartition being of such extent as to prevent communication between suchparts except such as may unavoidably occur through such interstices asmay exist between said bars, means for entering into said lower part ofsaid chamber a mixture of oxidized zinciferous material and reactioncarbon, a heating grid comprising graphite resistors in said upper partof said chamber, means suspending said grid in electrically insulatedrelation to the rest of the furnace and in spaced relation to saidpartition for heating said partition to a temperature at which thegraphite of which said bars are formed will react with carbon dioxide toreduce it to carbon monoxide and for causing said partition to conductand radiate heat downward on such mixture to produce zinc vapors andreaction gases including carbon dioxide, means for discharging suchvapors and gases from said lower part of saic chamber, said abuttinggraphite bars acting to reduce to carbon monoxide the carbon dioxide ofsuch of said gases as tend to pass from said lower to said upper part ofsaid chamber through the interstices between said abutting bars.

5. A zinc smelting furnace according to claim 4 in which parallelshelves are formed on a pair of the opposite side walls, respectively,of the re action chamber, the graphite bars being cylindrical androllingly resting at their opposite end portions on said shelves, thefurnace having provision for successively entering said bars into saidchamber and placing them on said shelves at one of the ends thereof andfor removing them fron said shelves to the exterior of the furnace atthe opposite ends of said shelves.

6. A zinc smelting furnace according to claim 4 in which parallelshelves are formed on a pair of the opposite side walls, respectively,of the action chamber, the graphite bars being cylindrical and rollinglyresting at their opposite end portions on said shelves, said parallelshelves be ing inclined to the horizontal, the furnace hav-- ingprovision for successively entering the bars endwise into the furnacefrom its exterior for placing them on the higher portions of saidshelves whereby successive bars so placed will roll down said shelves tobuild up the partition, and also having provision for successivelyremoving said bars endwise from the lower portions of said shelves tothe exterior of the furnace.

'7. An electric furnace having walls forming a furnace chamber, electricheating resistors in the upper portion of said chamber in electricallyinsulated relation to said walls for heating material entered into itslower portion, laterally abutting elongated bars of heat conductive heatre fractory material below said resistors in adjacent spaced relationthereto so as to be electrically insulated therefrom forming a partitionseparating the space containing said resistors from the space below it,said partition being of such extent as to prevent communication betweensaid spaces except such as may unavoidably occur through suchinterstices as may exist between said bars.

8. An electric furnace having walls forming a furnace chamber, electricheating resistors of graphite in the upper portion of said chamber inelectrically insulated relation to said walls for heating materialentered into its lower portion, laterally abutting elongated bars ofgraphite below said resistors in adjacent spaced relation thereto so asto be electrically insulated therefrom forming a partition separatingthe space contaming said resistors from the space below it, saidpartition being of such extent as to prevent communication between saidspaces except such as may unavoidably occur through such interstices asmay exist between said bars.

9. An electric furnace comprising walls forming a furnace chamber,electric heating resistors in the upper portion of said chamber forheating material charged to its lower portion, parallel shelves formedon a pair of opposite side walls, respectively, of said chamber, whichshelves are inclined to the horizontal, laterally abutting elongatedround bars of heat conductive heat refractory material forming apartition between the resistors and the lower portion of said chamber,which bars at their opposite ends have cylindrical portions thatrollingly rest on said shelves, the furnace having provision forentering said bars into said furnace from its exterior for placing themon the higher portions of said shelves whereby successive bars so placedwill roll down said shelves to build up said partition.

10. An electric furnace according to claim 9 in which a wall of thefurnace has an opening through which the bars may be slid endwise fromthe exterior of the furnace to place them on the shelves the higherportions of the latter, opposite walls of the furnace having openings atthe lower portions of said shelves for pushing and removing the barsendwise from said shelves to the exterior of the furnace.

Cited in the file of this patent UNITED STATES PATENTS Number Name Date950,904 Fitzgerald Mar. 1, 1910 1,105,538 Schwann July 28, 19141,110,359 Thierry Sept. 15, 1914 1,749,762 itsgerald Mar. 11, 19302,007,605 Heyroth et al. July 9, 1935 23171613 Poland June '2, 1949

